For mobile communication handset applications, there is a significant need for radio frequency (RF) filters to ensure appropriate frequency selectivity. As the world appetite for faster and higher quality wireless service has rapidly increased, the frequency spectrum for current wireless applications has become very crowded. As such, filter requirements have become ever more stringent. In certain instances, an RF filter must be capable of selecting immediately adjacent frequency ranges. This presents a significant challenge for filter technology.
Two general types of RF filters are commonly used: Surface Acoustic Wave (SAW) and Bulk Acoustic Wave (BAW). Of BAW filters, there are typically two types: Film Bulk Acoustical Resonator (FBAR) and Solidly Mounted Resonator (SMR). SAW and BAW filters both consist of coupled electro-mechanical resonators that convert electrical signals into acoustical waves of desired resonant frequencies. These resonant frequencies are chosen to give the desired bandpass frequency of the filter.
The response amplitude of an RF filter, as well as the filter's selectivity, are key factors in RF filter performance. Because the resonant frequency and resonance response of a RF filter is dependent of the quality of the piezoelectric material, having good crystal quality in the layers forming an RF filter is essential for device performance.